Correlation study between LeTID defect density, hydrogen and firing profile in Ga-doped crystalline silicon

Abstract

Light- and elevated Temperature-induced Degradation (LeTID) remains a challenge for the long-term stability of silicon-based solar cells. Despite numerous publications and studies on the subject, only a link with the hydrogen concentration in the silicon bulk has been established. However, the question remains as to how exactly hydrogen interacts with the defect or even forms part of it. In this study, highly sensitive resistivity measurements and photoluminescence/ photoconductance decay measurements are used to establish a precise correlation between the dominant hydrogen species (H2 and GaH) and the evolving defect concentration in fired Ga-doped silicon coated with an hydrogen-rich amorphous silicon nitride (SiN2:H). The correlation appears to be linear in both species over the range investigated but the data imply that the defect concentration is proportional to H2. An investigation of the behavior of GaH during typical degradation conditions confirms that GaH pairs are hydrogen sinks and therefore do not contribute to the formation of LeTID defects. We therefore expect the hydrogen dimer to act as a precursor for LeTID. In addition, the indiffusion of H during the firing step from a SiN2:H layer was investigated and a strong dependence on the peak temperature was found, whereas a weaker dependence on the cooling rate was observed.